The present invention relates to a semiconductor device, particularly to a semiconductor device with a field effect transistor constituted by an active layer of silicon semiconductor and its integrated circuit and a process for producing the same.
Conventionally, in a field effect transistor constituted by an active layer of silicon semiconductor, an MOS (Metal Oxide Semiconductor) structure using silicon oxide as a gate insulator is used.
To meet the requirements about high grade formation for the integrated circuit and reduction of operational voltage, it is necessary to reduce the transistor's sized and operate the transistor at high speed. In order to meet the requirements the miniaturization of the transistor's size as well as thin film formation for the gate insulator are needed. Conventionally, according to a gate insulator of silicon oxide which is most generally used in an integrated circuit of silicon, it has become apparent that when a film thickness thereof is made thinner than 3 nm, there poses a problem that tunnel current starts to flow from a gate electrode to a silicon active layer, a depletion layer of the silicon active layer becomes difficult to control and an increase in power consumption results.
As a means for solving the problem, attention is attracted to silicon nitride which is provided with a value of a dielectric constant approximately twice as much as that of silicon oxide, is constituted by amorphous, provided with stably high insulating performance and does not include elements contaminating a silicon semiconductor as a gate insulator substituting for silicon oxide. However, there poses a problem that a conventional silicon nitride thin film is provided with poor heat resistance and when the silicon nitride film is subjected to a heat treatment at 550° C. or higher after forming the silicon nitride film, an energy level of trapping or emitting carriers is produced and transistor characteristics are significantly deteriorated (‘P. J. Caplan, et al.: Journal of Applied Physics, 50 (1979) 5847’ and ‘F. B. Mclean: IEEE Transaction Nuclear Science, NS-27 (1980) 165’.